Æthelberht was the son of Eormenric, succeeding him as king, according to the Chronicle. He married Bertha, the Christian daughter of Charibert, king of the Franks, thus building an alliance with the most powerful state in contemporary Western Europe; the marriage probably took place before Æthelberht came to the throne. The influence of Bertha may have led to the decision by Pope Gregory I to send Augustine as a missionary from Rome. Augustine landed on the Isle of Thanet in east Kent in 597. Shortly thereafter, Æthelberht converted to Christianity, churches were established, and wider-scale conversion to Christianity began in the kingdom. Æthelberht provided the new mission with land in Canterbury not only for what came to be known as Canterbury Cathedral but also for the eventual St Augustine's Abbey.

Æthelberht’s law for Kent, the earliest written code in any Germanic language, instituted a complex system of fines. These can be found in the Textus Roffensis (circa 1120). Kent was rich, with strong trade ties to the continent, and Æthelberht may have instituted royal control over trade. For the first time following the Anglo-Saxon invasion, coins began circulating in Kent during his reign.

Æthelberht later was canonised for his role in establishing Christianity among the Anglo-Saxons, as were his wife and daughter. His feast day was originally 24 February but was changed to 25 February.